Download Overview of Custom Power Applications

Overview of Custom Power Applications
Narain G. Hingorani, Hingorani Power Electronics, 26480 Weston Drive, Los Altos Hills, CA 94022
I.
INTRODUCTION
Electric power is a form of energy we have come to depend
on, so much so, that in many automated product line
businesses can not tolerate its loss for even a few tens of
milliseconds. With the ever-increasing role of electricity in
improving the quality of life, productivity of manufacturing
and service industries, and efficient energy use, power
electronics will play a significant part. The Power
Electronics Building Block (PEBB) concept sponsored by
the Office of Naval Research (ONR) has played a key role in
accelerating the trend towards reduced cost, losses, weight,
volume, and much lower engineering effort to design
applications of power electronics. One of the growing power
electronics applications is the Power Quality/Custom Power.
This paper covers general aspects of the Custom Power
applications and a suggested scenario for a Custom Power
Park.
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No (or rare) power interruptions.
Magnitude and duration of voltage reductions within
specified limits.
Magnitude and duration of overvoltages within
specified limits.
Low harmonic voltage.
Low phase unbalance.
Acceptance of fluctuating, nonlinear and low power
factor loads without significant effect on the terminal
voltage.
This can be done on the basis of an individual, large
customer, industrial / commercial park or a supply for a
high tech community on a wide area basis.
Not all of the solutions call for Custom Power solutions,
customers with a few critical loads and small customers
would have a few low-power conditioning solutions.
III.
RELIABILITY CONSIDERATIONS FOR CUSTOMERS
II. CUSTOM POWER CONCEPT
Custom Power is a concept based on the use of power
electronic controllers in the distribution system to supply
value-added, reliable, high quality power to its customers.
For many customers this is a preferred alternative to the
customer improvising utility power by their own means,
mostly in a band aid manner with numerous uninterruptible
power supplies, as is done now. Many utilities are moving
in the direction of value-added Custom Power service to
their large customers.
Custom Power means that the customer receives specified
power quality from a utility or a service provider or at-thefence equipment installed by the customer in coordination
with the utility, which includes an acceptable combination of
the following features:
Narain G. Hingorani, Hingorani Power Electronics, 26480 Weston Drive, Los
Altos Hills, CA 94022
Until recently, voltage reductions (dips) and short term
outages were not an issue; the number of cumulative hours
of interruptions per year has been the benchmark measure
for reliability. Over the last ten years or so, the customers'
perception of reliability has been changing, and the pace of
change has accelerated. The concept of an outage time of a
few cumulative hours per year as being an extremely reliable
supply is no longer valid for an increasing number of
customers, industrial in particular.
An event of a few cycles interruption, or even of voltage
reduction to less than 80%, can cause big problems for
industrial customers. With the age of automation,
computers, process controls, drives and robotics, a new
concept of reliability must be developed. The number of
outages and voltage dips and their duration is much more
important than the cumulative outage time per year.
Hypothetically, if one cumulative hour is made up of 3,600
interruptions of a second duration each, it would be a
disaster for any automated industrial customer as compared
to one interruption of one hour. In reality a customer may
expect more than a hundred events of sags greater than 15%
lasting for less than a second.
Studies of cumulative monitoring of events at a large
number of distribution substations show that most of the sag
events are of short duration of a few cycles and less than
40% voltage reduction and only 1% of events with voltage
reduction of less than 80%. With the availability of two
independent feeders at the distribution substation, and highspeed solid state transfer switches, a large percentage (7090%) of the events in figure 2-2 can be eliminated in a much
more efficient and economical manner than the customers'
own solutions.
Custom Power is intended to protect the customers from
interruptions and voltage reductions originating in the utility
system as well as those transferred to customers from other
customers via the utility system and even internal
disturbances.
of a utility's transformers. These harmonics would then
be amplified by the natural resonances in the utility
system and/or the customer system.
There are a number of reasons for the choice of the Custom
Power concept for many customers, and on a long term basis
for most customers. These reasons are:
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IV.
NEED FOR CUSTOM POWER
The need for the Custom Power concept arises from the fact
that:
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Most of the interruptions and voltage reductions occur
in the utility system on account of lightning faults on
transmission and distribution lines, low frequency
dynamic swings of the transmission system, trees
touching the wires, equipment failure, switching, etc.
Voltage sags may also be a consequence of large load
changes affecting customers own equipment or affecting
other equipment via the utility system.
The customers are better served if they receive a
comprehensive solution to their power supply problems
from the power supply service providers.
Most of the voltage reductions and interruptions result
from events that involve the utility system.
In general, the total cost of the solutions that involve the
utilities' own systems would be much less than the cost
of solutions put in place by the individual customers.
At least from the point of view of reliability, migration
to the Custom Power concept seems to be inevitable.
Compared to voltage reductions, overvoltages are less
common and do not affect customer productivity unless the
overvoltages cause equipment failure. Nevertheless, they
represent an issue of safety and life of equipment. Switching
in and out capacitors with mechanical switches, or
regulating voltage with tap changers, does not provide the
coverage needed by, for example, the CBEMA limits. Nor
could the existing gapless zinc-oxide arresters ensure
overvoltage to less than 1.2 pu for 3 ms as specified in
CBEMA curve. Thus, power electronics-based voltage
control is essential for modern industry.
Impulses, switching surges and overvoltages affecting
the insulation, would most likely result from lightning
strikes and switching events in the transmission and
distribution system.
V.
Temporary overvoltages lasting from several cycles to
several seconds would largely result from large load
changes, capacitor switching, transformer switching,
dynamic stability swings, excessive leading-VARs
during light loads, etc. in the utility system.
One solid state switching solution involves the use of a bus
tie switch to share the load between two incoming feeders.
These two feeders should preferably come from different
transmission substations. However this is often not easy to
accomplish and the next best feeder service would be to
pick-up the two feeders for different transformers of a
transmission substation. The idea is that lightning faults do
not affect both feeders, or at least the effect on the two
feeders is generally unequal. The bus-tie switch is normally
open, and specific loads are assigned to the two feeders via
solid state circuit breakers. A fault on one feeder will lead to
opening of its circuit breaker. The bus-tie switch will close
to serve the loads from the other feeder as soon as the faulty
feeder is separated from the loads. This process can be
Voltage unbalances in a three-phase supply would occur
mostly due to large unbalanced loads on a utility's
distribution lines and long lines with unbalanced phase
impedances.
Harmonics would most likely be the consequence of
high harmonics in the customer load, or the saturation
PRINCIPAL CUSTOM POWER SOLUTION CONCEPTS
Figure 1 shows one-line diagrams of two basic distribution
substation solutions based on solid state switching.
completed in 4-8 ms. There are variations of this concept
including one in which the bus-tie switch is normally closed.
Another solid-state switching solution is that of two solidstate transfer switches, one open and the other closed, so
that the load is normally connected to one feeder, but is
transferred to the other feeder within 4 ms when there is a
voltage sag or distortion. If due to some common coupling of
the two feeders back in the system, the transfer will result in
load connection to the better of the two feeders. In both
solid-state switching solutions, the customer will be free
from the voltage dips for most if not all of the transmission
line fault and switching events. However the cost of transfer
switches would be much less than that of circuit breakers.
A more elegant approach is to combine the two converters,
the Statcom and the DVR, as shown in Figure 3. In this
arrangement, the shunt connected Converter can supply and
absorb reactive power in order to support the bus voltage,
supply and absorb the real power to support the series
connected converter, and act as an active current filter. The
series-connected converter on the other hand, supports the
customer voltage for any deviation, dips and interruptions.
The storage comes into play when the shunt connected
converter can not supply the power during large dips and
interruptions.
VI.
Figure 2 shows one line diagrams of two approaches to
converter-based solutions.
One converter-based solution, known as Statcom, involves a
shunt-connected voltage source converter, which basically
injects current into the ac system. The dc capacitor voltage,
through appropriate sequential switching, is converted to an
ac voltage, the difference between the converter generated ac
voltage and the system ac voltage results in current flow and
corresponding reactive power. DC storage means, such as
capacitors, storage batteries, or superconducting magnet,
connected through a power electronic interface such as a
chopper, can be utilized to supply real power as well and,
conversely its capacity can be used to absorb real.
Going beyond that, if the converter's internal frequency is
high enough, then it can simultaneously act as an active
current harmonic filter. The same converter can be given
multiple controls, such as ac bus voltage control by injecting
or absorbing reactive and active power, act as an active
current harmonics filter, filtering current as ordered to
absorb only the harmonics generated by the selected
customer loads, and even balance the phases. With large
enough storage, it can also serve as a backup power supply.
The other converter-based solution, known as the Dynamic
Voltage Restorer (DVR), which, through a series
transformer, connects the converter in series with the supply
voltage, and injects precise voltage in each phase, which
makes the load-side voltage free from voltage dips,
overvoltages, distortion and unbalance. DVR rated for
injection of +30% will take care of 90 percent of the dip
events and all dynamic overvoltages. It can be designed for
100% injection, in which case the DVR will act as a full
backup supply for as long as the storage system can supply
active power. The energy storage is connected with an
electronic interface to manage the active power required for
the voltage compensation. This DVR can also be designed to
serve as an active voltage filter, ensuring harmonic free
voltage for the customer.
TYPES OF LOADS AND POWER QUALITY REQUIREMENTS
In an unregulated environment, electric companies can offer
a variety of new value-added electric power products and
services in terms of quality and reliability of supply,
operation and maintenance of the customers' own electric
supply equipment, and also guaranteed performance. Given
the variety of customers and their needs and the utilities'
options, Custom Power would almost certainly be
customized for individual large customers, a group of
customers in an industrial park, or an office complex, etc.
Additionally, on-site, continuous monitoring of a number of
these Custom Power products and services will be necessary.
When any value-added service involves liabilities and
penalties against the promised service, the monitoring will
become somewhat more complex.
The process of agreement itself would call for an evaluation
of the sensitivity of the customers' production in relation to
the normal service, and how much value added expenditure
is justifiable. It is also likely that a high-tech industrial /
commercial area would have a normal wide area supply
service which is a cut above the normal, and then add
further value for the individual customer.
Within the context of the Custom Power Park, one would
expect to accommodate:
1) Small to medium size industries and commercial
organizations with load requirements in the range of
hundreds of KWs up to say about 10 MW.
2) Businesses that need a degree of improvement in the
reliability/quality of service.
3) Businesses that are willing to pay a premium in lieu of
the investor having to own and manage their power supply
conditioning equipment.
Effects of concern on a large variety of equipment include:
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Speed variation of motors/drives.
Stalling of motors/drives.
Damage to motors/drives.
Loss of computer data.
Misoperation of process control.
Failure of computers and electronic equipment.
Communication interference.
Overload of transformers, switchgear, cables, etc.
Misposition of robotics tool or machining operation.
Loss of cooling for a wide range of equipment.
Loss of lighting, elevators, air conditioning etc.
VII.
TYPES OF CUSTOM POWER SUPPLY SERVICES IN A
CUSTOM POWER PARK
In a Custom Power Park all customers will benefit from a
basic supply which is a cut above the normal power supply
from a utility. This basic supply can be obtained by
provision of two incoming feeders along with Solid State
Transfer switches (SSTs) which will reduce the duration of
most voltage dips to 4-8ms by rapidly selecting the feeder
with the highest voltage. It is also clear that most of the
customers generate moderate to high harmonics, and
therefore need an active filter service. Thus, the lowest grade
supply should involve use of two incoming feeders, SSTs
and an active harmonic filter.
As an example, the following loads are assumed for the
Custom Power Park. (Figure 4).
Max Load PQ Need (explained
below)
Semiconductor Chip Co. 10 MW CP-AAA
Biotech Co.
2 MW CP-AAA
Computer Hardware Co.
1 MW CP-A
Software Development Co. 1 MW CP-AA
Plastics Co.
3 MW CP-A
Hospital
10 MW CP-AA + CP-AAA
Data Processing Center
2 MW CP-AA + CP-AAA
Office Building
2 MW CP-A
Shopping Mall
2 MW CP-A + CP-AAA
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Total
33 MW peak load
In addition, there would be small common power needs for
parking facilities and driveways.
The key issue is how many grades of power should be
available, and how the choices are determined and managed.
Consideration of the value to different type of customers
suggests that three grades of qualities (CP-A, CP-AA, and
CP-AAA) with appropriate value-added premium cost, can
serve the need. As a minimum, all customers would receive
power CP-A with a quality superior to the regular power.
Custom Power Park Control system will play a key role in
the real-time and off-line management of services.
Custom Power A (CP-A): This is the basic value-added
power at the Park. Its higher quality, compared to the
regular power, arises from the fact that the Park has two
incoming feeders, which can be designed with improved
grounding, insulation and arresters, and better maintenance
which reduces the chance of nearby lightening and other
faults. With single phase tripping and reclose, the
interruptions and major voltage dips are greatly reduced. In
addition, the Solid State Transfer Switching ensures that the
feeder with superior voltage is selected in less than half a
cycle. There will still be voltage dips which are common to
both feeders resulting from faults and dynamic swings in the
transmission network. Dips from transmission line faults
usually last for 4-10 cycles, and occasionally the system
swings may cause 20-50 percent dips for about 1 second,
and then repeated if the power system is unstable. There is
also the possibility that a circuit breakers after clearing the
fault will reclose into a fault, and the voltage dip and system
oscillations will be repeated. Never the less, improvement
over and above the normal power is substantial and may be
characterized as follows:
1. 60-80 percent reductions in voltage dips.
2. Rare occurrence of interruptions.
3. Active harmonic filters.
Custom Power AA (CP-AA): Over and above grade CP-A,
grade CP-AA receives the benefit of a standby generator
which can come up to speed in about 10-20 seconds in case
of a power loss.
Custom Power AAA (CP- AAA): Over and above grade
CP-AA, grade CP-AAA receives the benefit of DVR, which
precisely adds the right amount of voltage including
harmonics to the feeder voltage to ensure virtually dip free,
interruption free, and harmonic free voltage to the customer.
It is assumed that under emergency conditions (loss of
DVR), CP-AAA may degrade to CP-AA. For unavailability
of backup power, it would still receive superior quality
power and priority customer service attention.
On the left hand side of the CP bus, Figure 4 is shown a
distribution bus for Custom Power A and AA. Both receive
the same service as long as either of the two feeders is
available. If and when both feeders are lost, the circuits of
all customers A and AA are disconnected at both ends by
their circuit breakers. The differentiating feature between
customers A and AA is that when the standby generator is
up and running (perhaps 10-20 seconds after the loss of both
feeders), the circuit breakers for customers AA are closed to
restore power to those customers. This means that customers
AA do not lose power for more than 10-20 seconds in the
case of loss of both feeders. However the customers A do
not receive power until one of the feeders is back in service
The standby generator normally stays off and disconnected
from the CP bus. When both feeders are off, the generator is
started up immediately, brought up to speed, synchronized
and connected to the CP bus. It should take 10-20 seconds
for the generator to come on line, and pick up the load of
both AA and AAA customers. It is reasonable to assume
that CP Control will supervise this fully automated dieseldriven or gas turbine generator of 5-10 MW size. When only
one feeder is off, CP Control will alert all the customers and
also the utility to confirm that the generator is available, has
adequate fuel supply, and to ensure that the utility is extra
careful in its system operation., the CP Control can in fact
start up the standby generator and bring it online.
Under peak load conditions and/or when one circuit off, the
CP Control may even bring the generator up and connected.
It can become a co-generator, even if both incoming feeders
are intact. Furthermore co-generation may be used as often
as economic conditions are favorable for co-generation
The active filter, can filter exactly the amount of harmonics
it is ordered to filter. Thus with current measurement on its
AAA customer side, it can ensure that the AAA customers'
harmonics are filtered. CP Control will supervise such an
operation, as well as monitor harmonics from other
customers. On the other hand, it can also sum the measured
current of select customers, and filter the harmonics of these
customers. The overriding considerations will be to ensure
that the active filter is not overloaded, harmonic voltage on
the CP-bus is not excessive and that the total harmonics
entering the feeders is reasonable. Because the harmonics
from various customers may cancel each other to some
extent, it may be best to order the active filter to minimize
harmonic content in the common busbar connection between
the two SSTs and the CP bus. As mentioned earlier,
practically all customer facilities generate harmonics and
need active filter support.
Each customer's waveform is monitored by the CP Control,
and all power quality events are captured, logged, and
managed for periodic assessment of the service being
provided. It is also necessary to evaluate specific events for
continued improvement, and collect the information
required
to
resolve
disputes,
performance-based
payments/penalties, etc.
It is obvious that there can be a variety of scenarios for a
Custom Power Park. The scenario presented in this paper
provides the basis for further discussion.
VIII
In order to ensure that the CP- AAA customers are not
deprived of power during the period that the generator is
brought up to speed and on line, and to supply them power
without dips, distortion and imbalance, a combination of
Active Filter, Dynamic Voltage Restorer, and hot standby
storage is provided as shown on the right hand side of
Figure 4.
In addition, connection to the AAA customers can also be
made through solid state current limiting circuit breakers
(SSCL). This can ensure that the fault current resulting from
faults in the CP-AAA customers own system does not
exceed, say, 2 times their peak load for about 1 or 2 seconds
before tripping. This will further ensure that other CP-AAA
customers are not affected. SSCLs can provide coordination
with the downstream protective relays of the CP-AAA
customers. These SSCLs also limit over loading on the
DVR. Another alternative would be to provide a saperate
series converter (DVR) for each customer or a CP superAAA customer, etc.
REFERENCES AND ACKNOWLEDGEMENT
[1] N. G. Hingorani, "Introducing CUSTOM POWER"
IEEE Spectrum June 1995.
Author acknowledges the support the support of Terry
Ericsen of ONR and David Flamm of SRI for supporting the
conceptual definition of the Custom Power Park.
Feeders
Feeders
Circuit Breakers
Transfer
Switches
Bus Tie
Loads
Loads
Loads
Figure 1. Custom Power based on Feeder Switching
Feeder
Load
Solid State
Circuit Breaker
Feeder
Load
Converter
Converter
Interface
Interface
Storage
Storage
Figure 2. Custom Power based on Voltage Source Converter
Feeder
Active Filter
DVR
Converter
Converter
Load
Interface
Storage
Figure 3. Custom Power based on two Voltage Source Converters and Storage
CP Control
C ustom
Power
Park
Substation
C ustom
Power
Park
Figure 4. Custom Power Park Concept